The present invention relates to an electrostatic printing apparatus comprising an improved printing electrode head.
U.S. Pat. No. 3,898,674 to Koch discloses improvements to a non-impact printer which is disclosed in U.S. Pat. No. 3,550,153 to Haeberle. In this printer, a pulsed electric field is applied between a shaping and base electrode through a donor sheet and a closely adjacent recipient sheet to transfer electrically conductive printing material particles from the doner sheet to the recipient sheet. A shield electrode is disclosed by Koch which cancels the fringe components of the pulsed electric field emanating from points on the surface of the shaping electrode outside the printing face in order to reduce the size of the printed mark and increase the resolution of the system. An important aspect of this prior art printer is that a gap must be maintained between the donor sheet and the recipient sheet.
The present invention constitutes an improvement to an electrostatic printer which eliminates the need for a donor sheet and thereby the cost of printing. In such a printer, a rotary cylinder is coated with a liquid or dry layer of a developing substance comprising charged particles of pigment or the like and a recording sheet, which may be an ordinary sheet of writing paper, is fed in contact with the cylinder. A printing head is positioned closely adjacent to the cylinder so that the recording sheet is disposed between the cylinder and printing head. The printing head comprises at least one shaping electrode which may be in the form of an alphanumeric character. Alternatively, a number of shaping electrodes may be provided in a dot matrix configuration. A voltage applied to the shaping electrode or electrodes attracts adjacent particles to the recording sheet for printing, with the particles being later thermally or otherwise fixed to the recording sheet to provide a permanent record.
A problem which has heretofore remained unsolved in this type of system is that since the developing substance is maintained in contact with the recording sheet charged particles will adhere to the recording sheet even in non-printing areas. This creates a smeary appearance and severely limits the contrast which can be provided by the printer.